The orientation of tail fins in fish and marine mammals is influenced by their respective modes of swimming.

Fish, with their horizontal body movement achieved by sweeping their fins left and right, have vertical tail fins. This vertical orientation allows them to effectively capture the water and generate propulsion during swimming. However, it’s worth noting that flatfish like flounders, which lie on their side, can be an exception to this general pattern.

On the other hand, marine mammals, including whales, have evolved from land-dwelling mammals that exhibit a galloping motion on land, characterized by a vertical undulation of the spine. When these mammals transitioned to an aquatic lifestyle, they retained this vertical motion. As a result, their tail fins, known as flukes, have a horizontal orientation to facilitate efficient swimming and propulsion in the water.

Therefore, the orientation of tail fins in fish and marine mammals is directly related to the specific swimming techniques and adaptations developed by each group throughout their evolutionary history.